Obvious part: people differ in whom they are attracted to. More to the point, there is considerable variation in specific qualities in another desired person driving each person's sexual arousal and attraction. (Original thought: sex is fun. Exactly what about sex makes it fun? The answers will differ a lot from person to person.)
Profound part: Therefore, sex drive must be learned behavior; it must be taught. If it were innate, we would expect less variation in what we are attracted to. Very roughly, there just aren't enough genes to encode the observed variation in preferences.
Of course, without sex drive, humans would quickly go extinct. After making the profound observation, we muse at how it's surprising that humans haven't gone extinct already, having at some point in some sort of dysfunctional society failed to effectively teach sex drive to the next generation. I suppose it is similar to how some baby animals need to be taught what to eat by their parents; they do not instinctively eat what tastes delicious.
Giant questions: what are the mechanisms by which sex drive is taught and learned? What causes a person to become sexually attracted to which qualities?
These mechanisms must be very robust -- or else extinction. I suspect they form a huge part of how society functions, including manifesting in conflict. Our ability and predilection to learn sex drive is likely innate; I suspect our predilection to teach it is also instinctual, which means that a huge part of how society functions is hardcoded instinctual behavior. This places limits on how much we can change society for political or altruistic reasons: instinct is very powerful -- or else extinction.
The answers to the giant questions seem gettable. It's kind of surprising the answers aren't known already, given how important the questions are. Crowdsourcing might be useful. Techniques in analyzing Big Data might be useful. However, the fact that it is a taboo topic might (continue to) impede things.
With this model, it is easy to explain the existence of preferences and behaviors commonly called sexual deviancy, some seemingly paradoxical in being not conducive to producing babies: people simply learned them. (However, with all sexual preference being learned, it's hard to call learning some things normal and others deviant. The only preference that might fairly be called normal is learning no sex drive at all: only deviancy saves us from extinction.) Understanding what specific environmental experiences cause people to learn which sexual preferences is still work to be done: they are the giant questions above. (We can see how this is a taboo topic: having understood the mechanisms by which someone became sexually deviant, we can then place blame for the deviancy on the people around them doing or allowing those mechanisms, as well as absolve of guilt criminally sexually deviant behavior.)
This model is scientifically testable, in principle: after hypothesizing a mechanism by which people learn some sexual preference, some aspect of sex drive, we can do experiments exposing one group to the mechanism and another group not. Unfortunately, such experiments seem highly unethical and very difficult to do in a controlled manner.
We can also continue to test the other side of the nature versus nurture debate: look for correlations or lack of correlation in sexual preferences between genetically related individuals. Of course, this is tricky: genetically related people often live in very similar environments. We can again imagine highly unethical experiments.
What evolutionary advantage was there for humans to have this indirect method of acquiring sex drive? Probably something to do with tribes. Is it unique to humans?